1. Field of the Invention
This invention relates to the field of vibration Analysis.
More particularly, this invention relates to transducers to detect and produce optical representations of vibrational energy.
In still greater particularity, but without limitation thereto the invention will be described as it pertains to an interferometric vibration sensor.
2. Description of the Prior Art
Much of the initial enthusiasm surrounding the application of optical fiber technology to sensor systems was generated by the potential for high sensitivity using interferometry and the possibility of constructing a completely optical system which takes advantage of fiber optic telemetry. The need for parallel electrical systems to electrical systems to power and condition the electronic outputs of the vibration sensors is thereby eliminated. The promise of high sensitivity was not based on the intrinsic sensitivity of optical fibers to physical stimuli but on the fact that existing demodulators can resolve optical phase shifts on the order of microradians. Additionally, optical path lengths of tens or hundreds of meters could be incorporated in sensors of modest physical dimensions. The combination of high interferometric demodulator resolution and long optical path length led to the possibility of displacement measurements with resolutions on the order of a part in 10.sup.11 to 10.sup.14. In addition to measurement of conventional physical parameters such as temperature, pressure, etc., fiber optic sensors were suggested for such unusual measurements as the detection of cosmologically generated gravitational waves.
With time it became apparent that such high sensitivities might not lead to practical devices since microradian optical interferometric demodulators were not easy to produce outside the laboratory. In addition, the optical fiber was found to be as sensitive to other extraneous physical parameters as it was to the parameter it was designed to detect. The concomitant optical phase shift resulting from extraneous parameters made it difficult to stabilize a "reference fiber" in one arm of a conventional interferometer against fluctuations in ambient conditions such as temperature, static pressure, accelerations, etc.